In photolithography as used for manufacturing semiconductor devices, liquid-crystal display devices, thin-film magnetic heads, and the like, a projection-exposure apparatus (e.g., a "stepper") is generally used to project a pattern, defined by a photomask ("mask") or reticle (either referred to generally below as a "reticle"), onto a sensitive surface of a semiconductor wafer, glass plate or other suitable substrate (referred to below as a "wafer"). The surface of the wafer is "sensitive" because it is coated with a photoresist or other suitable light-sensitive composition in a manner analogous to a photographic emulsion. Such projection-exposure apparatus typically include a projection-optical system (comprising a projection lens and illumination-light optics) that must exhibit excellent image-formation characteristics in order to transfer an image of the pattern image of the reticle onto the wafer with high resolution.
To such end, image-formation characteristics such as image-surface inclination and curvature of field of the projection-optical system are periodically measured. Correction of certain image-formation characteristics is usually performed based on the measurement results.
The projection-exposure apparatus generally include wafer stages comprising a Z-tilt stage that can adjust the wafer's axial position (height) and inclination in the Z direction and an XY stage that positions the wafer at a desired position on the XY plane.
Based on the premise that the wafer stage, especially the running guideways of the wafer stage, is nearly perfectly flat, measurement of the image-formation characteristics of the projection-optical system is conventionally performed as shown in FIGS. 16 and 17, wherein FIG. 17 shows a portion of the XY plane of FIG. 16. In both figures, measurements are taken of certain aspects of the image surface (on the wafer 3B) in the exposure field IAR (indicated by dashed line in FIG. 17) of the projection-optical system 1.
In FIG. 16, a focal-point-detection device 50 (comprising a light transmitter 50a and a light receiver 50b) is operable to detect the focal point of a wafer 3B situated downstream of the projection-optical system 1. In FIG. 16, the Z axis is parallel to the optical axis AX of the projection-optical system 1; the X and Y axes are on a plane perpendicular to the Z axis. A detection beam FL, produced by the light transmitter 50a, propagates at an angle to the optical axis AX toward a fixed detection point C located on or near the optical axis AX on the wafer 3B. The light transmitter 50a transmits a slit image onto the detection point C. The detection beam FL is reflected by the wafer 3B and enters the light receiver 50b. The light receiver 50b produces an output signal corresponding to the height position of a measurement point P1 on the wafer 3B corresponding to the detection point C. As shown in FIG. 16, there is a swell in the surface S of the wafer 3B. To prevent the swell from affecting the image-surface measurement, exposure is performed in the following sequence:
(1) The XY stage 5 is actuated to move the measurement point P1 on the surface S of the wafer 3B to the detection point C, and the Z-tilt stage 7 is actuated as required to place the measurement point P1 at the desired location (at the detection point C).
(2) With the Z-tilt stage 7 in a fixed position, the XY stage 5 is actuated to position the measurement point P1 at a measurement location A located at one end of the exposure field IAR. (In FIG. 16 the curved broken line shows the wafer position before actuation of the XY stage 5, and the curved solid line shows the wafer position afterward.) As shown in FIG. 17, wafer movement causes the area AR1, illustrated with broken lines on the wafer 3B that correspond to the exposure field IAR, to shift to the exposure field IAR. With the wafer 3B then remaining fixed, an illuminating light flux, generated by an illumination light source (not shown) located upstream of the projection-optical system 1, is irradiated onto a test pattern defined on the reticle (not shown, but located upstream of the projection-optical system and downstream of the illumination light source). The image IPA of the test pattern is exposed onto the measurement point P1 on the wafer 3B. In this scheme, the test-pattern image IPA is made up of multiple slit marks extending in the X direction and multiple slit marks extending in the Y direction (FIG. 17).
(3) The Z-tilt stage 7 is then actuated and the actions described in (1) and (2) above are repeated at multiple wafer elevations (Z-dimension positions) by changing the height of wafer 3B. The previously exposed test-pattern image IPA and the newly exposed test-pattern image IPA on the wafer do not overlap because the exposure position on the wafer 3B is slightly displaced in the XY plane each time as a result of moving the wafer each time in the Z direction.
(4) The actions described above in (1)-(3) are repeated at multiple locations other than position A on the wafer in the exposure field IAR.
(5) The exposed wafer 3B is developed; the exposed pattern reveals the position in the Z direction yielding the best image resolution for each position inside the exposure field IAR.
By the actions in (1)-(5), it is possible to determine how much curvature and inclination exists with respect to the image-formation surface of each position within the exposure field IAR, with the running guideway of the XY stage 5 as the reference.
In the conventional technique described above, a measurement error with respect to image-formation characteristics can arise if the flatness of the XY stage 5 running guideway is not sufficiently precise. This is especially because the XY stage 5 moves until the action described in (2) performs an exposure only after the action described in (1) establishes (measures) the height of the measurement point P1. There is a practical limit to the flatness of the running guideways of the XY stage 5. Also, the flatness tolerance of the running guideways of the XY stage tends to increase over time after repeated use. Also, as the numerical aperture of projection-optical systems increases so as to allow for further refinement of semiconductor elements and the like made using projection exposure, the required depth of focus has been decreasing.
Therefore, since the requirements for image formation, such as the image-surface curvature and inclination of the projection-optical system, have become more stringent, it is now impossible to ignore errors with respect to image-formation characteristics that accompany insufficient flatness of the XY stage running guideways. This problem cannot be avoided when using conventional methods and apparatus.
Moreover, since it is necessary to repeat the actions described above in (1)-(3) for each of the multiple measurement positions inside the exposure field IAR, the amount of time it takes to do the measurement increases in proportion to the number of measurement positions. This disadvantageously results in reduced throughput (productivity).
Furthermore, projection-exposing the reticle pattern onto the wafer involves an operation in which the patterned surface of the reticle is aligned with the wafer surface; that is, the focal points are aligned. As described above, depth of focus has recently been decreasing in projection-optical systems. Even when an i-line wavelength (365 nm) is used for exposure illumination, the depth of focus is only about 0.7 .mu.m. In addition, the trend is toward greater increases in the size of the projection field of the projection-optical system.
Reference is made to U.S. Pat. No. 5,118,957 in which, in order to achieve satisfactory focal-point alignment across a wide exposure field, a slit image is projected obliquely relative to the optical axis AX (i.e., not projected through the projection-optical system) onto each of multiple measurement points inside the shot region (exposure field) of a wafer. A focal-point detector system (multiple-point AF system based on the oblique-incidence method) is used to receive and analyze light from each reflected image by a two-dimensional array sensor.
Reference is also made to U.S. Pat. No. 5,502,311 (corresponding to Japanese Laid-Open patent document no. Hei 5-190423) disclosing a separate optical system used for detecting focal points of the reticle pattern versus the wafer. The '311 patent also describes a method for calibrating such a multiple-point AF system with respect to the best-focus image surface at the time of projection exposure.
The multiple-point AF system disclosed in the '311 patent does not include a separate optical system for each measurement point. Rather the '311 patent discloses embodiments in which a "pin-hole" image from a light source is projected through a common optical system onto the wafer surface. The image directed by the optical system onto each measurement point on the wafer surface is reflected from the wafer surface and directed by the optical system to a detector. Each reflected image is separately detected using a two-dimensional sensor array in the detector.
According to the prior art, calibration of a multiple-point auto-focus (AF) system involves a determination of any offset between respective sensors corresponding to each measurement point. Correction of the offset between sensors involves servo control of the Z stage in consideration of each offset.
The focus calibration method described in the '311 patent uses a TTL (through-the-lens; i.e., through the projection-optical system) focus detection system and an oblique-incidence type multiple-point AF system to individually determine the offsets at each measurement point. An approximate plane is identified by means of the least-squares method using the determined offsets. The offsets of each measurement point are corrected and calculated with an approximate plane as a new reference. Thereafter, good focusing is achieved across the entire shot region (exposure field) on the wafer by vertically moving and/or tilting the Z stage so that the output signals obtained at each measurement point are equal to the offsets previously corrected and calculated.
It has been discovered, however, that focus can be blurred when attempting to expose a reticle pattern in a shot region after aligning the Z position of the shot region on the wafer using an oblique-incidence-type multiple-point AF system as summarized above in which focus calibration is performed by finding individual offsets at each measurement point. Moreover, careful investigation has revealed unstable circumstances in which the degree of focus disparity characteristic of a wafer having a certain type of exposure layer differs from the degree of focus disparity characteristic of a wafer having a different type of exposure layer. Also, it has been found that the focusing precision obtained with the oblique-incidence type multiple-point AF system varies with the wafer type.